Light permeable resin, generally called “optical n” or “optical polymer” is employed as a material for film-like, plate-like, lens-like or other optical member used in various optical apparatuses (for example, film, substrate, prism sheet or the like used in liquid crystal displays; lenses in lens system for signal reading of optical disk; fresnel lens or lenticular lens or the like for projection screen).
Birefringence is one of important optical characteristics to be considered when an optical member is formed of an optical resin. That is, it is generally undesirable that an optical member has a large birefringence. Specifically, in the usages as exemplarily referred to as above (liquid crystal displays, optical disk device, projection screen), any film, lens or the like arranged in a light path gives bad influence on image quality or signal reading performance, and accordingly an optical member showing birefringence as small as possible is demanded. Doubtlessly, small birefringence is desirable for lenses for cameras, glasses or the likes.
Any way, as well-known in the field, birefringence of optical polymer (called simply “polymer” hereafter as required) includes birefringence caused mainly due to orientation of principal chains, namely “orientation birefringence”, and birefringence caused mainly due to stress, namely “photoelasticity birefringence” (usually called simply “photoelasticity”). Orientation birefringence and photoelasticity birefringence have signs, respectively, each of which depends on chemical structure of polymer and is characteristic of individual polymer.
In other words, orientation birefringence generally appears when principal chains of chain-like polymer (polymer chains) are orientated, and this orientation of principal chains is generated in drawing or extruding molding process for producing polymer film, or injection molding process frequently employed for producing optical members of various shapes, namely in processes involving material flowing, and the generated birefringence remains in an optical member.
On the other hand, photoelasticity birefringence is birefringence which is caused by an elastic deformation (distortion) of polymer. An elastic deformation (distortion) remains in an optical member formed of polymer when any elastic deformation (distortion) is generated, for example, in the material by volume shrinkage involved by temperature reduction from a temperature about the glass transition temperature to another temperature lower than the glass transition temperature, providing cause of characteristic orientation birefringence.
Further, if an optical member is mounted, for example, to an optical device used in normal, temperature (lower then the glass transition temperature), any external force applied thereto also causes the material to be deformed elastically, thereby bringing photoelasticity birefringence.
As known well, movement of principal chains of optical polymer is generally frozen even if any elastic deformation occurs under a temperature not higher than the glass transition temperature, and orientation state of the principal chains are kept substantially unchanged. Therefore, it will can be say that photoelasticity birefringence emerges based on mechanism different from that based on which orientation birefringence merges, viewed from microscopic or molecule-level points,
As known well, orientation birefringence and photoelasticity birefringence have signs, respectively, and some kinds of polymers show orientation birefringence and photoelasticity birefringence signs of which are inverse to each other (i.e. orientation birefringence of plus-sign and photoelasticity birefringence of minus-sign, or orientation birefringence of minus-sign and photoelasticity birefringence of plus), which suggests that there is  difference between emerging mechanism of orientation birefringence and that of photoelasticity birefringence.
As described above, orientation birefringence and photoelasticity birefringence emerge based on different mechanism, and orientation birefringence and photoelasticity birefringence shown by conventional optical resins are various, but it is difficult to find an optical resin showing orientation birefringence and photoelasticity birefringence both of which are sufficiently small.
For example, although polycarbonate and polystyrene are low-cost and have high transparency and high refractive index, they show orientation birefringence and photoelasticity birefringence both of which are large, bringing a disadvantage.
Theoretically saying, orientation birefringence does not emerge if no orientation is involved by production processes of an optical member including a molding process applied to an optical resin. Actually, prior arts have tried improving various conventional molding methods in order to reduce orientation birefringence by suppressing orientation of polymer intensively as possible in cases of molding processes for lens, film or the like.
For example, in a case of injection molding, employed are method according to which temperature of polymer melting is heightened or polymer temperature is kept relatively high within a mold. Acceding to another method for producing film, polymer is dissolved in a solvent and the obtained polymer solution is developed on substrate and then the solvent is eliminated by drying.
Indeed suppressing of polymer orientation is achievable to an extent as described above, but suppressing of orientation gives a generally reduced production velocity as compared that of production without suppressing orientation.
Further saying, an improvement in order to prevent photoelasticity birefringence from emerging has been applied. For example, in cases of injection molding or extruding molding where an optical member is produced from melting state, volume of polymer occurs shrinkage during a temperature reduction from melting state to normal temperature, with the result that distortion is generated by stress and photoelasticity birefringence is raised. Therefore, heating at a temperature lower than a certain temperature for several hours to several ten hours in order to remove distortion. Of course, such a process added reduces production efficiency and bring an economical disadvantage.
In addition, even if distortion is removed, any external force is applied during being used, photoelasticity birefringence emerges, which gives a shortage not overcome.
Although researches for reducing birefringence of optical resin by adding an additive have been carried out and some of them have been reported, every art aims to make only one of orientation birefringence and photoelasticity birefringence of an optical resin of base material of an optical member roughly zero by cancelling said one by orientation birefringence or photoelasticity birefringence of inverse sign of the additive.
According to orientation cancelling methods, a monomer of plus sign orientation birefringence and another monomer of minus sign orientation birefringence are copolymerized, or an organic compound of low molecular-weight (low molecular-weight organic compound) is added. Academic articles report that such methods are applicable for cancelling photoelasticity birefringence.
However, according to tow methods above, addition concentration of the low molecular-weight organic compound or copolymerization composition of copolymer which is required to deaden and almost eliminate orientation birefringence is largely different from a value required to deaden and almost eliminate photoelasticity birefringence, failing to almost eliminate both orientation birefringence and photoelasticity birefringence.
Concretely seeing, in the first place, Non-patent Document 1 noted below discloses “Method of cancelling birefringence by copolymerization”. According to this method, birefringence of polymer chains is cancelled by performing copolymerization randomly at an appropriate ration between a monomer composing homopolymer showing plus orientation birefringence (i.e. monomer of plus orientation birefringence) and another monomer composing homopolymer showing minus orientation birefringence (i.e. monomer of minus orientation birefringence). Non-patent Document 1 shows benzyl methacrylate as a monomer of plus orientation birefringence and methyl methacrylate as a monomer of minus orientation birefringence and, which are copolymerized. Further, it is shown is that orientation birefringence is almost eliminated at a weight-ration 82/18 for benzyl methacrylate/methyl methacrylate, and that photoelasticity birefringence is almost eliminated at a ration 92/8 for benzyl methacrylate/methyl methacrylate,
Next, Patent Document 1 noted below discloses an invention relating to “Method of cancelling birefringence by adding a low molecular-weight organic compound”. According to this invention, a low molecular-weight organic compound to a polymer resin having plus or minus orientation birefringence, wherein the low molecular-weight organic compound has orientation birefringence of sign inverse to sign of the polymer resin so that orientation birefringence of the low molecular-weight organic compound cancels that of the polymer and aiming to obtain a non-birefringent optical resin material.
In an embodiment thereof, trans-stilbene is added to polymethyl methacrylate to cancel birefringence of polymethyl methacrylate. Orientation birefringence of polymethyl methacrylate containing trans-stilbene added thereto is almost eliminated when addition concentration of trans-stilbene is 3.0 wt %.
Next, Non-patent Document 2 noted below discloses “Method of cancelling photoelasticity birefringence of polymer by adding a low molecular-weight organic compound”, which is similar to the method disclosed in Patent Document 1 referred to above.
According to an example thereto which is similar to the embodiment disclosed in Patent Document 1, trans-stilbene is employed as an additive to be added to polymethyl methacrylate.
According to reported data in the example, photoelasticity birefringence of polymethyl methacrylate containing trans-stilbene added thereto is almost eliminated when addition concentration of trans-stilbene is 2.2 wt %, while orientation birefringence of polymethyl methacrylate containing trans-stilbene added there to is almost eliminated when addition concentration of trans-stilbene is 3.0 wt %, which tells that both values of addition concentration are largely different.
In connection with the present invention, it is noted that Non-patent Document 3 noted below gives description of infrared-dichroism method which is a well-known method and utilized for measuring degree of orientation of principal chains of copolymer molecules or homopolymers, as referred to later.    Non-patent Document 1; Shuichi Iwata, Hisashi Tsukahara, Eisuke Nihei, and Yasuhiro Koike, Applied Optics, vol. 36, p. 4549-4555 (1997)    Non-patent Document 1; Tokkai-Hei 8-110402 (JP)    Non-patent Document 2; H. Ohkita, K. Ishibashi, D. Tsurumoto, A. Tagaya, and Y. Koike, Applied Physics A, published online on Dec. 21, 2004.    Non-patent Document 3; Akihiro Tagaya, Shuichi Iwata, Eriko Kawanami, Hisashi Tsukahara, and Yasuhiro Koike, Jpn. J. Appl. Phys. vol. 40, p. 6117-6123 (2001)